1. Field of the Invention
The present invention generally relates to the performance of cardiac ablation in order to eliminate abnormal heart rhythms or arrhythmia. More specifically, the present invention relates to a relatively non-invasive, phased array ultrasound cardiac ablation system and method which is capable of compensating for acoustic aberrations between the array and the cardiac tissue. The system also compensates for movement of the treatment volume during the ablation procedure.
2. Description of the Prior Art
Currently, a variety of treatments are available for abnormal heart rhythms, hereinafter arrhythmias, such as atrial fibrillation, supraventricular arrhythmias, ventricular fibrillation, ventricular tachycardia, bradycardia and others. These treatments include both surgical and non-surgical treatments. The non-surgical treatments are principally through the use of anti-arrhythmic drugs while the surgical treatments involve implantable devices and ablation of cardiac tissue.
Anti-arrhythmic drugs slow the intercardiac impulses which sustain the arrhythmia once started. These drugs decrease the likelihood that an arrhythmia will occur. Atrial fibrillation, the most common arrhythmia, is often treated in this manner. As well documented in the literature, currently available anti-arrhythmic drugs exhibit undesirable side effects and can prove fatal in extreme cases. For this reason, surgical alternatives are often used and preferred.
Until recently, implantable cardiac defibrillators have been the surgical treatment of choice for life threatening arrhythmias. An automatic, implantable cardioverter defibrillator (ICD), is used to shock the heart and stop an ongoing arrhythmia. After the shock, normal sinus rhythm resumes. ICDs have become an accepted treatment for ventricular arrhythmias. These arrhythmias generally do not respond to drug treatments. Another device, "Pacemakers", as they are generally known, are programmable implantable units that stimulate and control excessively slow cardiac rhythms through a series of electrical impulses.
Both pacemakers and ICDs respond to the electrophysical basis of ventricular arrhythmias. Neither, however, corrects the root cause of the arrhythmia and for this reason is possible for the arrhythmia to recur. The ICD functions by discharging a high voltage capacitor which is conductively connected to the heart. The energy required for successive discharges requires that the device's battery be periodically replaced. The implantation of both of the above devices requires surgery and, with the ICD, the total cost of the device and the implantation is in the $50,000 dollar range. In addition to its high cost, with an ICD, a constant expectation of an unpleasant "shock" sensation remains with some patients. To some, this poses a significant psychological burden.
Another surgical treatment for arrhythmia, tissue ablation, actually corrects the underlying electrophysiological cause of the arrhythmia. Tissue ablation generally involves the transmission of energy to a selected portion of cardiac tissue to ablate the tissue. In some situations, tissue ablation is utilized as an adjunct therapy for patients with implantable defibrillators. Some arrhythmias, however, are not amenable to ablation because of limitations in the techniques currently being used. One example is ventricular tachycardia.
Current developmental ablation techniques use a variety of energy sources including direct current (DC) energy; radio frequency (RF) energy; microwave energy; cryothermic energy; and laser energy.
In direct current, myocardial tissue ablation, a common catheter is inserted into the heart and 2,000 to 4,000 volts of electricity are applied over several milliseconds. Ablation according to this technique is performed under general anesthesia due to the severity of the muscular contractions associated with the electrical shock used during the procedure. Damage to the catheters used in delivering these high voltages has been seen and, as a result, the generation of an electrical discharge at a non-intended site within the patient is possible.
RF ablation of myocardial tissue is similar in that it is a catheter based technique which induces tissue damage to eliminate the arrhythmia. With RF ablation, 40 to 60 volts of energy are used to thermally treat the desired tissue. One significant limitation on the use of RF ablation techniques is that the low energy generation and the significant dissipation of this energy after delivery result in the size of the ablated area being very limited. The treatment is also limited to those areas which can be reached by a catheter based RF probe.
Having all the general limitations associated with RF ablation techniques, microwave ablation techniques are similarly limited. Additionally, microwave energy tends to be difficult to focus. This is because of the relatively long wavelengths of the frequencies believed necessary for ablation.
Catheters having cyroprobes on their end have also used to ablate cardiac tissue. Perforation of the cardiac tissue is a danger with this method since the temperatures required to adequately perform ablation (-78.degree. C.) require that a large catheter tip is used.
Laser ablation techniques seem to hold some promise, but some concerns remain regarding tissue perforation, equipment deterioration, equipment durability and portability.
With all of the above described systems, ablation is discussed as being performed through a fully invasive method. In each system, the source of the energy required for performing ablation is applied via a catheter which is inserted into the patient to the appropriate treatment area through a venous or arterial route. The procedures are also tedious and do not always allow for the catheter to be placed as close as necessary to the tissue in need of treatment. Non-invasive systems are an attractive alternative.
Catheter based ultrasonic transducers have also been proposed for ablating cardiac tissue. Although not available commercially, single and phased array transducers have been suggested in the relevant literature. An electrode also associated with the distal end of the catheter is used to electrically map the conduction pattern in the heart. The electrode aids in positioning and orienting a transducer relative to the target tissue. The transducers typically generate frequencies in the 1-40 Mhz range.
Like the other energy based ablation techniques mentioned above, the catheter based ultrasound technique is an invasive procedure. Additionally, manufacturing a phased array transducer of a size capable of being mounted to the end of a catheter, is currently not practical because of the number of transducers involved and because of the necessary size of the array required for adequate ablation.
In designing a relatively or completely non-invasive system, one where the source of the ablation energy is external of the patient, requires that aberrations created by tissues in the treatment "window" overlying the heart must be considered. Movement of the heart itself must also be considered when delivering energy from an external source.
If the above are overcome, relatively or wholly non-invasive procedures have the potential of increasing overall quality of the treatment as well as increasing the number of patients that can be treated and reducing cost. Cost savings would be realized not only from the surgical techniques themselves, but also by decreasing patients being treated with implantable defibrillators and by "curing" patients already having implants, as opposed to merely arresting an occurring arrhythmia.
In view of the foregoing limitations and shortcomings of the prior art devices, as well as other disadvantages not specifically mentioned above, it should be apparent that there still exists a need in the art for a relatively or wholly non-invasive cardiac ablation system and method.
It is therefore a primary object of this invention to fulfill that need by providing a cardiac ablation system and method which operates in a relatively or wholly non-invasive manner.
Another object of this invention is to provide a cardiac ablation system and method which can perform the ablation procedure as a wholly non-invasive procedure.
Still another object of this invention is to provide an apparatus and method whereby cardiac ablation is available to treat atrial fibrillation, supraventricular arrhythmias, ventricular fibrillation, ventricular tachycardia and bradycardia.
It is also an object of the present invention to provide a cardiac ablation system and method which is capable of focusing on a specific cardiac tissue treatment volume.
A further object of this invention is to provide a cardiac ablation system and method which can correct for significant aberrations in the treatment window between the energy emitter and the heart of the patient.
Still another object of this invention is to provide an apparatus and method for cardiac ablation where movement of the treatment volume is compensated for during the performance of ablation.
A further object of this invention is to provide an apparatus and method for cardiac ablation which is capable of making large lesions and ablating relatively large treatment volumes.
Still another object of this invention is to provide a system and method for performing cardiac ablation while using ultrasonic energy.
Another object of this invention is to provide an apparatus and method for cardiac ablation which utilizes a phased ultrasound array located externally of the patient.